A speed changing unit of the type including clutches at a first stage (on the auxiliary speed changing side) and clutches at a second stage (on the main speed changing unit side) as viewed from an input shaft thereof is adapted to select a certain speed stage using a proper combination of clutches on the auxiliary speed changing unit side and clutches on the main speed changing unit side.
Hitherto, a speed changing unit of the aforementioned type is so constructed that clutches on the auxiliary speed changing unit side and clutches on the main speed changing unit side are simultaneously brought in an engaged state by simultaneously feeding hydraulic oil to them when speed changing is effected. Namely, when such a conventional system as mentioned above is employed, hydraulic oil is simultaneously fed to two clutches at the time of speed changing.
Thus, the conventional system has a drawback that hydraulic pressure of hydraulic oil delivered from the hydraulic pump is lowered due to a shortage in amount of hydraulic oil delivered therefrom compared with a capacity of the respective clutches when an amount of hydraulic oil delivered therefrom is reduced at the time of starting of forward movement, speed changing under the influence of inertia or the like occasion. As shown in FIG. 10, the lowering of hydraulic pressure of hydraulic oil delivered from the hydraulic pump causes delayed filling time and fluctuation in filling time, resulting in a variety of malfunctions such as generation of shock due to speed changing or the like taking place when filling is terminated, for instance, in the course of gradual increase of hydraulic pressure for the clutches in a case where the latter are intended to be controlled with the same pattern of hydraulic pressure.
Hence, the present invention has been made with the foregoing background in mind and its object resides in providing a method and an apparatus for controlling a speed changing unit which assure that a malfunction attributable to lowering of hydraulic pressure of hydraulic oil delivered from a hydraulic pump when an amount of hydraulic oil delivered therefrom is reduced at the time of starting of forward movement or the like is obviated.